1. Field of the Invention
This invention pertains to new carbon black and rubber products and more particularly to improved carbon blacks that can be incorporated into elastomers for manufacture of tire rubber compounds characterized by low hysteresis and reduced rolling resistance.
2. Description of the Prior Art
Carbon blacks arc produced in great quantities throughout the world. They are commonly used as reinforcing agents for elastomers, and as fillers and/or colorants for plastics, paints and printing inks. The largest use for carbon black is to impart strength and wear resistance to the elastomer compounds employed in the manufacture of pneumatic tires for motor vehicles and aircraft. In tires, carbon black greatly improves the abrasion resistance and traction of the tread rubber, but can also affect the hysteresis of the rubber compound and hysteresis of the tires. This latter property relates to the loss of energy that occurs as a result of deformation of a tire as it rolls over a surface such as a road or highway. A tire formed from a rubber compound with a low hysteresis value will have a reduced resistance to rolling, which is beneficial.
Carbon blacks are formed in special furnaces wherein a fuel is burned in a combustion zone with excess oxygen that is typically supplied as a stream of air or as an air/oxygen mixture. Within a reaction zone of the furnace, carbon black feedstock, which may be in the form of a highly aromatic, high molecular weight hydrocarbon oil, is commingled with the highly heated, oxygen-containing combustion gases from the combustion zone, and the carbon black feedstock is partially burned, thermally decomposed and dehydrogenated within the reaction zone to produce an aerosol comprising particulate carbon black suspended in the gaseous products of reaction. The aerosol is cooled by quenching with water and the carbon black particles are separated from the remaining gases and vapors and collected. The resultant carbon black particles, which are collected as a fluffy powder, can be densified by use of conventional pelletizing methods.
During the carbon black formation process, the carbon black particles form aggregates that loosely cling together as "soft" agglomerates. As seen by means of an electron microscope, the carbon black aggregates appear as spherical particles fused together to form a cluster. Commercial grades of carbon black are identified by the size and physical/chemical characteristics of the aggregates as measured by ASTM testing procedures which are well known to those skilled in the art.
The surfaces of the carbon black aggregates comprise carbon atoms in both amorphous and crystalline forms. FIG. 1 depicts a carbon black aggregate where the darker areas represent the crystallites and the lighter areas correspond to the amorphous carbon. Amorphous carbon has no crystalline or organized structure. Within crystallites, however, the carbon atoms are arranged in an organized fashion and exist in planar layers having well defined edges. These crystallites typically have an average width of about 25 .ANG. and are approximately 12 .ANG. thick. The extent of edge exposure presented by the planar edges of the crystallites, and more specifically, the extent to which the conduction electrons at the crystallite edges are able to interact with other materials, such as rubber during compounding and curing, is a measure of the "surface activity" of the carbon black. Carbon blacks that have surfaces that are too "active" tend to disperse poorly in elastomers, and will not exhibit optimum performance upon cure of the rubber compounds that contain the carbon black. The terms "surface activity," "activity," and "active," as used herein, refer to the interactive nature of carbon black to form its own network and interact with an elastomer as the result of crystallite presence on the surface of carbon black aggregates as described herein.
Due to the aforementioned problems that can be associated with carbon blacks having a high surface activity, what is needed are carbon blacks with a reduced degree of surface activity whereby dispersion in elastomer compounds is improved and production of new and improved vulcanizates can be achieved.